Polycystic ovary syndrome (PCOS) is a common endocrine condition that affects women of reproductive age. Anovulation, menstrual irregularities, hirsutism, and infertility are common clinical presentations. Long-term health concerns such as type II diabetes mellitus and, possibly, cardiovascular disease, have been linked to PCOS. Metformin, an oral hypoglycemic agent, has been recently advocated as treatment for some women with PCOS due to the association of PCOS with hyperinsulinemia. Metformin is utilized as sole therapy for ovulation induction as well as in combination with traditional ovulation-induction therapies. This review identified 23 prospective studies addressing the effects of metformin on PCOS. Because of the heterogeneity of the published reports, only a qualitative assessment of the data was possible. Review of this literature confirms a beneficial role of metformin in reducing insulin resistance in some women with PCOS. Other favourable biochemical effects include reduced free testosterone levels and increased sex hormone-binding globulin (SHBG). Metformin may improve menstrual regularity, leading to spontaneous ovulation, and improve ovarian response to conventional ovulation-induction therapies. There is, however, little evidence supporting the use of metformin to facilitate weight reduction, or improve serum lipids or hirsutism. Further evaluation is required to define the long-term effectiveness of metformin, who will benefit from metformin treatment, and the optimal duration of metformin therapy.